1. Field of the Invention
The present invention relates to Mnxe2x80x94Zn ferrite and a coil component using Mnxe2x80x94Zn ferrite as a magnetic core, and in particular to Mnxe2x80x94Zn ferrite and a coil component suitable for switching power supplies, noise filters, choke coils and so forth.
2. Description of the Related Art
Magnetic core materials for use in noise filters such as line filters for EMI countermeasures are desired to have high impedance in a frequency band where noises should be removed. Impedance in a high frequency band can be increased through the phenomenon of resonance between the stray capacitance (C) of a coil and the inductance (L) of a core. It is known that impedance is maximum at the resonance frequency (1/LC). It is also known that impedance at a frequency lower than the resonance frequency is proportional to the initial permeability of a core, and impedance at a frequency higher than the resonance frequency is inversely proportional to the stray capacitance of a coil.
Mnxe2x80x94Zn ferrite that has high permeability in a wide frequency band has been studied intensively and disclosed in, for example, Japanese Patent Laid-open Nos. 2000-353613 and Hei 9-180925. The initial permeability of Mnxe2x80x94Zn ferrite is closely related to a real part xcex5xe2x80x2 of complex relative permittivity, and when xcex5xe2x80x2 has a large value, the initial permeability is relaxed in a high frequency band and cannot be maintained through up to a high frequency band. As a result, it is difficult to obtain excellent impedance properties in a high frequency band.
While Japanese Patent Laid-open No. 2000-353613 mentioned above does not describe dielectric properties or impedance properties, Japanese Patent Laid-open No. Hei 9-180925 refers to permittivity (relative permittivity) xcex5, and describes the relative permittivity xcex5 as reading 50,000 to 1,000,000 at 1 KHz, which is very high. Usually, complex relative permittivity xcex5 is expressed as (xcex5xe2x80x2-jxcex5xe2x80x3), and although the above-mentioned relative permittivity xcex5 is not separated into a complex component and a real component, generally, in case of polycrystalline Mnxe2x80x94Zn ferrite, xcex5xe2x80x3/xcex5xe2x80x2=0.5 to 1.5. Accordingly, xcex5xe2x80x2 may be estimated to range from 30,000 to 60,000 at the lowest, which is a very high value. The real part xcex5xe2x80x2 of complex relative permittivity with such a high value at 1 kHz not only can never be 50 or less at 1 MHz, but also causes relaxation of initial permeability in the vicinity of 500 kHz making it impossible to maintain the initial permeability through up to a high frequency band of 1 MHz or more.
The present inventors have found that a coil component comprising a magnetic core of conventional Mnxe2x80x94Zn ferrite having a high real part xcex5xe2x80x2 of complex relative permittivity scarcely reduces stray capacitance despite whatever optimizations of its winding conditions. The stray capacitance of a coil component is roughly classified into capacitance between coils and capacitance between a coil and a core. The former is determined based on the winding conditions of a coil and the latter is determined based on the material properties of a magnetic core, both increasing as the real part xcex5xe2x80x2 of complex relative permittivity increases. Therefore, in the coil component with a magnetic core of conventional Mnxe2x80x94Zn ferrite having a high real part xcex5xe2x80x2 of complex relative permittivity, the capacitance between a coil and a core is extremely high as compared with the capacitance between coils. As a result, the optimization of winding conditions has been hardly effective in reducing the stray capacitance of the component.
When designing a magnetic core material and a coil component to be used in a high frequency band, two requirements must be met to reduce stray capacitance: one is to use a magnetic core material that maintains high permeability through up to a high frequency band and the other is to optimize its winding conditions. The coil component using conventional Mnxe2x80x94Zn ferrite as its magnetic core satisfies neither of the two requirements, making it difficult to obtain high impedance in a wide frequency band.
The present invention has been made in consideration of the above-mentioned conventional problems, and an object of the present invention is therefore to provide Mnxe2x80x94Zn ferrite that maintains initial permeability over a wide frequency band, has small stray capacitance when provided with a coil, and has excellent impedance properties over a wide frequency band, and also a coil component made thereof.
In Mnxe2x80x94Zn ferrite according to the present invention, basic components include 44.0 to 50.0 mol % (50.0 mol % excluded) Fe2O3, 4.0 to 26.5 mol % ZnO and the remainder MnO, and its real part xcex5xe2x80x2 of complex relative permittivity is 20,000 or less at 1 kHz and 50 or less at 1 MHz, respectively.
The Mnxe2x80x94Zn ferrite of the present invention may further contain as additive at least one of 0.01 to 4.0 mass % SnO2 and 0.01 to 3.0 mass % TiO2 (provided that an upper limit is 4% in total when both thereof are contained). The Mnxe2x80x94Zn ferrite of the present invention may also contain as additive at least one of 0.01 to 2.0 mass % CuO, 0.01 to 2.0 mass % NiO, 0.01 to 2.0 mass % CoO, 0.01 to 2.0 mass % MgO, 0.01 to 2.0 mass % Al2O3 and 0.01 to 2.0 mass % Cr2O3 (provided that an upper limit in total is 2 mass % when two or more thereof are contained).
A coil component according to the present invention uses the above-described Mnxe2x80x94Zn ferrite as a magnetic core.
The Mnxe2x80x94Zn ferrite and the coil component according to the present invention can provide good impedance properties over a wide frequency band and effectively convert noises at a practically critical frequency band of 10 to 100 MHz into thermal energy and absorb them. Thus, they prove to be very useful.
FIG. 1 is a graph illustrating frequency properties of impedance of the present invention samples and comparative samples.